To fix this error just follow the steps to ensure that no duplicates are created in your Xero account.
1. Please login to your Xero account.
2. Find the item and 'void' it.
3. Login to Dext Prepare and find the item in your Inbox (you can try the 'Advanced search' engine too).
4. Select the item from the tickbox and click on the 'Tools' menu at the top to choose 'Clear publishing data' option before exporting.
5. Re-publish the item from your 'Inbox' by clicking the 'Publish' button.